A muffler generally includes a housing having at least one exhaust inlet and at least one exhaust outlet. Pipes may protrude into the housing on the inlet end and on the outlet end, and it is essentially possible to provide at least two pipes leading in parallel into the housing and/or out of the housing on the inlet end or the outlet end. In the case of mufflers, generally the problem is that gas flows against a flow resistance through the muffler, thus creating an increase in pressure in the exhaust system upstream from the muffler. This increase in pressure can have a considerable negative effect on the performance and efficiency of the internal combustion engine in certain operating states. It is therefore fundamentally possible to open and close at least one of the parallel pipes by means of a corresponding control device as needed. The switchable pipe is closed at low rotational speeds and/or at a low load of the internal combustion engine, whereas the pipe is opened at a higher rotational speed and/or at a higher load. The flow resistance of the muffler can be greatly reduced by adding-on the switchable pipe as described here. However, it has been found that this adding-on operation may be associated with additional noise emission into the environment because the silencing effect of the muffler is fundamentally optimized with regard to operating states having a low exhaust flow.
It is known (for example, German Patent DE 197 43 446 A1) that the exhaust system of an internal combustion engine can be designed with two flows, i.e., with two separate exhaust lines through which flow can pass in parallel, in at least one section. Each of these exhaust lines then has its own muffler. The two mufflers have different damping characteristics, i.e., they are tailored for different frequencies or frequency ranges. In addition, one of the exhaust lines may be opened and closed by means of a control valve. In the case of a low exhaust flow, the switchable exhaust line is blocked so that only the one muffler has exhaust flowing through it. When there is a high exhaust flow, the switchable exhaust line is opened so that exhaust passes through both mufflers.